On the Rotation of Post - T Tauri Stars in Associations

Nearby associations or moving groups of post-T Tauri stars with ages between ∼10 and 30 Myr are excellent objects for the study of the initial spin up phase during the pre-main sequence evolution. An empirical approach is adopted here for the first time with these stars to infer their rotations properties and relations to X-ray emission. Three nearby associations with distances less than 100 pc are considered. The TW Hya association (TWA) with an age of 8 Myr, the Beta Pictoris moving group (BPMG) with an age of 12 Myr and a combination of Tucana and Horologium associations Tuc/HorA (30 Myr). Two low and high rotation modes are considered for each association with stellar masses of 0.1 ≤ M < 1.5 M⊙ and 1.5 ≤ M ≤ 2.6 M⊙ respectively. Because no observed rotational periods are known for these stars, we use a mathematical tool to infer representative equatorial rotation velocities v0(eq) from the observed distribution of projected rotational velocities (vsini). This is done for each mode and for each association. A spin up is found for the high rotation mode, whereas in the low rotation mode the v0(eq) do not increase significantly. This insufficient increase of v0(eq) is probably the cause of a decrease of the total mean specific angular momentum for the low mass stars between 8 and 30 Myr. However, for the high mass stars, where a sufficient spin up is present, the specific angular momentum is practically conserved in this same time interval. A two dimensional (mass and vsini) K-S statistical test yields results compatible with an spin up scenario. By supposing that the distribution of masses of these three associations follows a universal mass function, we estimate the number of members of these associations that remain to be detected. The analysis of rotation and stellar masses using the luminosity X-rays indicators Lx and Lx/Lb present similar properties, as the dependence on [email protected] [email protected]